An operating gas circulation type internal combustion engine is known which uses an inert gas such as argon as the operating gas (i.e., heat medium), combusts hydrogen in a combustion chamber, condenses water vapor in the exhaust gas and discharges it outside the system, and supplies the exhaust gas from which the water vapor was removed (i.e., the inert gas) to the combustion chamber again. Japanese Patent Application Publication No. JP-A-11-93681 describes an internal combustion engine which supplies oxygen gas including argon as an impure gas from an oxygen supply apparatus to a combustion chamber via an intake port and injects hydrogen into the combustion chamber. Moreover, this internal combustion engine is structured discharge some of the exhaust gas from which water vapor has been removed through condensation outside the system so as to discharge argon of an amount equivalent to the amount of argon supplied into the system as well as oxygen from the oxygen supply apparatus out of the system. Accordingly, the concentration of argon in the gas supplied to the combustion chamber via the intake port is able to constantly be maintained at a substantially fixed concentration.
If the operating state of the internal combustion engine (for example, the load expressed by the operating amount of the accelerator pedal) changes such that the torque required from the internal combustion engine (hereinafter simply referred to as “required torque”) changes, the amount of hydrogen to be combusted in the combustion chamber changes. As a result, the amount of oxygen supplied to the combustion chamber also changes.
Therefore; the inventors researched the change in thermal efficiency of the internal combustion engine when the amounts of hydrogen and oxygen are changed while the amount (i.e., the flowrate) of argon is kept constant in a gas of hydrogen, oxygen, and argon as the operating gas supplied to the combustion chamber (hereinafter this gas will be referred to as “mixed gas”) (that is, when the concentration of argon in the mixed gas is changed). FIG. 1 is a graph showing the results.
As can be understood from FIG. 1, maximum thermal efficiency of the internal combustion engine is achieved when the argon concentration in the mixed gas is a value D0. In the range where the argon concentration is less than the value D0 it is estimated that the thermal efficiency decreases because the heat generated by the internal combustion engine is not as easily transmitted to the argon the lower the argon concentration. Also, in the range in which the argon concentration is higher than the value D0 it is estimated that the thermal efficiency decreases because combustion becomes unstable due to the relative decrease in the oxygen concentration in the mixed gas the greater the argon concentration. The value D0 of the argon concentration, which results in the thermal efficiency of the internal combustion engine, fluctuates depending on the amount of heat generated in the combustion chamber and the combustion state. In other words, the value D0 fluctuates according to the amounts of hydrogen and oxygen supplied to the combustion chamber which change according to the required torque. Misfire will occur if the argon concentration exceeds a value D1 (which is greater than D0).
However, the internal combustion engine described above only keeps the ratio of argon to oxygen supplied to the combustion chamber constant regardless of the required torque. Therefore, the argon concentration is unable to be maintained at a value that corresponds to the required torque (i.e., that corresponds to the amounts of hydrogen and oxygen supplied to the combustion chamber). As a result, the thermal efficiency of the internal combustion engine may decrease due to the argon concentration becoming too low, or due to combustion becoming unstable from the argon concentration becoming too high.